In processing a workpiece, an apparatus for generating plasma of a processing gas by a microwave is used. An example of such a apparatus is disclosed in Japanese Patent Application Laid-Open Publication No. 2014-175051. The apparatus disclosed in Japanese Patent Application Laid-Open Publication No. 2014-175051 includes a waveguide, a circulator, and a matcher.
The waveguide includes a first end and a second end. The circulator includes a first port, a second port, and a third port. The second port is coupled to the first end of the waveguide, and the third port is coupled to the second end of the waveguide. The matcher is provided between an input end of the microwave and the first port of the circulator. In addition, a slot hole extending in a propagation direction of the microwave is formed in the waveguide. In this apparatus, the microwave from the input end is propagated from the first port to the second port, and then, the microwave propagated to the second port is propagated from the first end of the waveguide to the second end of the waveguide. The microwave propagated to the second end of the waveguide is returned to the matcher from the third port of the circulator via the first port. A part of the microwave returned to the matcher is reflected by the matcher to be propagated to the first port of the circulator. In this apparatus, a standing wave is suppressed from being generated in the waveguide. That is, in order to generate plasma, a travelling wave is generated in the waveguide. In addition, since the microwave supplied from the input end and the microwave reflected from the matcher are supplied to the waveguide, a power density of the microwave becomes high in the slot hole. In this apparatus, the processing gas supplied to the slot hole is excited by the microwave, and the plasma is generated.
In the apparatus disclosed in Japanese Patent Application Laid-Open Publication No. 2014-175051, in order to more stably generate the plasma in the slot hole, it is desired to further increase the power density of the microwave in the slot hole.